Field of the Invention
The present invention relates to a radiation imaging apparatus, a radiation imaging system, and a control method for a radiation imaging apparatus.
Description of the Related Art
Radiation imaging apparatuses generate an image based on a difference between an image corresponding to a state with radiation emitted (hereinafter, referred to as a radiation image) and an image corresponding to a state with no radiation emitted (hereinafter, referred to as a dark image). It is difficult to generate an accurate image without taking information corresponding to the dark image into consideration, especially when amorphous silicon is used for photoelectric conversion elements that are disposed in the radiation imaging apparatus and convert information on light into information on electrons. The dark image is highly correlated with the temperature in the radiation imaging apparatus. More specifically, a rise in the temperature tends to result in a larger number of dark image components. An amplification circuit that integrates electrical charges, obtained with the radiation, has a characteristic also highly correlated with the temperature. Thus, generally, the characteristic is unstable due to temperature drift for a certain period of time after power is supplied to the amplification circuit.
Thus, in some radiation imaging apparatuses using amorphous silicon, after the power is supplied, a warming-up operation is performed to achieve a predetermined temperature in the radiation imaging apparatus and a predetermined temperature of the amplification circuit, so that a state with a predetermined number of dark image components and with the temperature drift reduced can be achieved. However, taking the time required for achieving the state with a predetermined number of dark image components and with the temperature drift reduced is extremely inconvenient in a case where the radiation imaging apparatus is required for emergency purposes, such as in an emergency hospital. On the other hand, an image formed of a difference between the radiation image and the dark image, wherein said dark image is not obtained in the state with a predetermined number of dark image components, due to a reduced the temperature drift.
Thus, in recent years, a method has been employed in which the warming-up operation is performed with the radiation imaging apparatus constantly turned on, so that the radiation imaging apparatus is ready to be immediately used, as discussed in Japanese Patent Application Laid-Open No. 2011-101693. In Japanese Patent Application Laid-Open No. 2011-101693 discusses a state where a voltage applied to the photoelectric conversion elements and thin-film transistors for reading the electrical charges accumulated in the photoelectric conversion elements is set to be equal to the ground potential. This state is referred to as a sleep state in Japanese Patent Application Laid-Open No. 2011-101693. However, it is difficult to use the radiation imaging apparatus in the state with a predetermined number of dark image components for the emergency use due to the following reason. Generally, in the radiation imaging apparatus, a predetermined voltage is applied so that electrical charges are accumulated in the photoelectric conversion elements. Thus, in a configuration where the voltage described above is set to be equal to the ground potential in the sleep state and then is suddenly raised to be applied to the photoelectric conversion elements for the emergency use, a certain period of time is required for achieving a state with predetermined electrical charges in the photoelectric conversion elements. In such a period, reliable image data is difficult to acquire due to the varying of the number of dark image components.
In another available method for general radiation imaging apparatuses, voltages applied to the photoelectric conversion elements and the thin-film transistors are controlled through the warming-up operation with a timing that is the same as that in a normal read drive, while the amplification circuit is being driven. However, the method involves a shift in a threshold voltage of the thin-film transistors, causing a change in the characteristic. Furthermore, the warming-up operation might be performed for quite a long period of time, and such a long period results in the threshold largely shifted. The shift in the threshold leads to varying of components leaked from the transistors, adversely affecting the image.
With the control method using the sleep state, as discussed in Japanese Patent Application Laid-Open No. 2011-101693, the temperature drift of the amplification circuit can be reduced, but it is difficult to maintain the threshold of the thin-film transistors at a constant value, while preventing the number of dark components from varying, which inevitably occurs when a sensor substrate is used, for the emergency use.